Micro-accelerometer



April 6 P. L. CONTENSOU ETAL 3,438,267

MI GRO-ACCELEROMETER Filed March 28, 1966 Sheet of 8- INVENTOQ5:

91mm; ONTEN HE.L .DELAT HEL J.GAY

ATTORNEY- mac-M April 15, 1969 P. L. CONTENSOU ETAL MICRO-ACCELEROMETERSheet Z of 8 Filed March 28, 1966 INvEMToRs: mam: COMTEMSOU, MICHELUELATTRE, MICHEL GAY ATTORNEY: gap/1,54

April 15, 1969 Filed March 28, 1966 P. L. CONTENSOU ETALMICRO-ACCELEROMETER Sheet 3 of 8 FIGA ' mvmmns:

PIERQF. CONTENSOU MICHEL DELATTR E.

MIC NEIL GAY nrroamm: a. 4.1

Sheet INVENTOFPfi: sou, RE

MICRO-ACCELEROMETER PIERRE commane-4. DELATT MICHEL cTAv ATTORNEY: 4 41%P. L. CONTENSOU ETAL April 15, 1969 Filed March 28, 1966 April 6 P. L.CONTENSOU ETAL 3,438,267

MICRO- ACCELEROMETER Filed March 28, 1966 Sheet 5 r sf IHVENTORS:PIEQQE. CONTEHSOU,

MICHEL DELATTRE, MICHEL GAY ATTORNEY:

April 15, 1969 P. L. CONTENSOU ETAL 3,4

HICRQACCELEROMETER I Filed March 28, 1966 Sheet 038 new mvawrons; PIERREONTENSOU, MICHEL ELATTQE. mcuen. GAY

ATTORNEY: a. d-

April-1 5, 1969 P. L. CONTENSOU ETAL MICROAGCELEROMETER Sheet Z FiledMarch 28, 1966 FlG.l2

JIM/M0000:

pen-0mg cancer/w mmmm | |i- PRORAHMER mvcmom= PIERRE COHTENSOU, MICHELDELATTRE,

MICHEL I GAY April 15,1969 P. L. coNTENsoQ ETAL 3,438,267

MICRO-ACCELEROMETER Sheet Q 018 Filed March 28, 1966 8 VIM/MONO! DETEC70R PROGRAMMER l H I 31 CORktCf/NG NETWORK LMREU/NG alrrmm/lt AMPL mmalfft'kmrm 231' cmp cmya AHPL/HER INVEN O S ONTE NSOU Pismaa c vucaez.DELATTRE MICHEL GAY ATTORNEY.- Q,

United States Patent US. Cl. 73517 8 Claims ABSTRACT OF THE DISCLOSURE Amicroaccelerometer of the electrostatic type providing an integratedacceleration measurement along 3 orthogonal axes comprising a hollowcasing, a spherical sensor floating in the casing in a gravity-freecondition when not subjected to acceleration, a plurality of opposedpolar electrodes isolated from the casing in orthogonal relation theretocorresponding to the orthogonal axes, the polar electrodes formingcapacitors by Virtue of the connection with the spherical mass of thecasing, and means for applying AC current between the casing andelectrodes with receiving means for the output signals from thecapacitors, the detected and amplified output signals indicating thedisplacement of mass of the casing from the orthogonal axes lyingbetween the circumpolar electrodes to the capacitors.

This invention relates to a micro-accelerometer of ball type and,specially, a high sensitivity, three-dimensional micro-accelerometer ofthis type.

The object of the invention is to provide very high accuracymicro-accelerometers capable of measuring accelerations of the order ofto 10 g such as those accelerations applied by solar radiation pressureon satellites.

Ball accelerometers generally consist of a ball and a casing, means forsensing the position of the ball in the casing and means for controllingthe position of the ball, these position controlling means beingcontrolled by the position sensing means. Ball accelerometers in whichthe sensing means are of electrostatic or optical or magnetic type andin which the position controlling means are either magnetic or hydraulichave already been proposed. In the case of the previously indicatedsensitivities, it is not possible to use balls made out of a magneticmaterial which react to the earths magnetic field. Furthermore, it isnot possible to link the ball to the casing by a flexible connectionwire so as to fix the potential and the latter must be completely freeand fioating in its casing.

Another object of the invention is to provide a microaccelerometer inwhich the ball position sensing means and the ball position controllingmeans are both electrostatic.

Generally speaking, the micro-accelerometer concerned by the inventionconsists of a floating spherical ball, that is to say, a ball which isnot suspended and not linked to a wire, three electrostatic differentialposition sensors located round the ball and in its immediate vicinityand arranged to transmit signals representing the 3,438,267 PatentedApr. 15, 1969 movements of the ball along three orthogonal axes with thecenter of the ball at the origin when the latter oc' cupies a particularposition termed a balanced or at rest position, three ball positioningelectro-static circuits located round the latter and in its immediatevicinity and arranged so as to produce mechanical action of the ballalong the said three orthogonal axes, means for separately amplifyingeach of the signals transmitted by the position sensors and means forapplying these to the positioning circuits whilst acting in the samedirection as that of the position sensor transmitting the signal.

The position sensors and the position control devices are electrostaticor capacitative, that is to say consisting of capacitors the firstelectrode of which is the ball itself (which, of course, is a conductor)with the second electrode installed on the casing of the apparatus. Thecasing is substantially spherical and the diameters of the casing andthe ball differ to a very slight extent. Electrodes insulated from theremainder of the casing are inserted in the spherical casing. There aretwelve electrodes six of which, shaped in the form of spherical caps,are located around the poles where the three orthogonal axes whoseorigin is at the center of the spherical casing meet said casing withthe six other electrodes consisting of spherical rings or segmentssurrounding the spherical caps and insulated from the latter. The poleelectrodes are ball position sensing electrodes and the circumpolarelectrodes are ball position control electrodes. The pole electrodes aresupplied with alternating current and form, together with the conductingball when the latter moves under the elfect of acceleration forces, avariable capacitor with a capacitance depending on the position of theball. Since the ball is free in the casing, its potential is fixedcapacitatively through a capacitor which is formed by its own surfaceand the surface of the casing not occupied by the polar and circumpolarelectrodes (the latter surface being taken as a potential referencesurface), this capacitor being relatively large, and, as a result,having relatively low impedance. The signals received between two givenand diametrically opposite polar electrodes are amplified and applied inthe same suitable direction between the two circumpolar electrodesassociated with said two given pola electrodes. A three component ballposition follower system is thus formed with the signals applied to thethree pairs of circumpolar electrodes respectively measuring theacceleration components applied to the ball.

As has just been explained, the fixed electrodes on the casing areportions of a spherical surface, cap (spherical segment with a base) orring (spherical segment with two bases). The capacitances which are thusobtained are, for small values of the radius of the base of the cap orof the radius of the large base of the ring, larger than the capacitancewhich would be obtained between the ball and a fiat electrode even inthe case where this flat electrode would be infinite.

If R is the radius of the ball e the spacing between the concentric balland easing, h the height of the cap and r the radius of the base of thespherical cap, the capacity between the sphere and the said sphericalcap is where s is the permittivity of the vacuum or, more generally ofthe medium in the space existing between the casing and the ball.

The capacitance between the ball and an infinite plane at a distance ofe from the ball is (refer to C. Snow, Formulas for Computing Capacitanceand Inductance, National Bureau of Standards, Circular 544, Sept. 10,1944):

For R=20 mm. and e=5.10"" mm., Formula 2 gives C =9 pf. Assuming thate-l-e', such a value of C is obtained for r=3.6 mm.

It can thus be ascertained that, relatively to the ball, a cap with abase measuring a few millimeters can replace a large flat electrode.

The accelerometer concerned by the invention has a threshold sensitivitylevel of the order of 10- g and is designed to effect measurements ofacceleration under conditions close to those of weightlessness and to bemounted on space vehicles.

Another object of this invention is to make the sensitivity of themicro-accelerometer vary as a function of the acceleration which itmeasures.

Another object of the invention is to provide means for calibrating themicro-accelerometer.

In a terrestrial laboratory, the micro-accelerometer is subjected to thegravitational field. It is, therefore, necessary that the measuringrange of the micro-accelerometer be from g to 10 g, that is to say 10times the sensitivity threshold, that the stability of the response he10 times the measuring range and that it be possible to subject theaccelerometer to accurately definite accelerations having amplitudeswhich would lie between g and 10 g. These three conditions are extremelydifiicult to fulfill in practice at the present stage of technicalprogress.

The calibration of the micro-accelerometer is effected by placing it ina free-falling capsule, preferably in a vacuum tube. A state ofweightlessness is thus simulated and this simulation improves with theextent of the vacuum in the tube since the friction engendered by theremaining air on the capsule results in a deceleration which couldreduce the accuracy of the simulation. In order to achieve a simulationof weightlessness of the order of 10- g, a vacuum of the order of 10*mm. of mercury must be available. The duration of the simulationexperiment depends on the height of the tube. The applicant has a vacuumcolumn approximately 40 meters high thereby resulting in an experimentaltime of 2.8 seconds.

Calibration time is divided into two parts. Between initial instant tand an instant t the accelerometer is placed in an operational statewithin its measuring range, that is to say, the ball, which isoriginally at rest in the casing, is led into the space in which it issubjected to the effect of the electrostatic positioning device. Betweeninstant t and the end of the calibration measurement time T, theaccelerometer and the capsule which contains it is subjected to anacceleration of known amplitude compatible with the measuring range.

More specifically, at instant (t e), preceding the instant of release,the accelerometer is subjected to an acceleration g and, at instant (t-i-e) immediately following the instant of release, it is subjected to aremaining acceleration of less than g. At instant (t e), the ball shapeis distorted under the effect of its own weight and is maintained in itsposition by the reaction of the casing. During time 26, this stress isfreed and the ball gains ascending speed relative to the support. Inorder to effect calibration, it is necessary that the ball be led, inthe time (t -t which is only a fraction of the duration of free-fall(T-t within the measuring range (range in which the follower system isoperative) and that its residual kinetic energy be less than that whichthe follower forces can apply thereto and which correspond to the limitsof the measuring range. This condition will be termed the ballpre-positioning condition. During the period (T -1 the capsule carryingthe micro-accelerometer is subjected to a pre-determined accelerationwhich results in an acceleration for the microaccelerometer lying withinthe measuring range.

The ball pre-positioning condition, is obtained by means of thefollower,systems controlling the electrostatic position control devices.

According to a feature of the invention, the microaccelerometercomprises three follower systems receiving input signals consisting ofthe electrostatic position detector signals and producing output signalsapplied to ball position control devices, at least one resistance andcapacitor circuit, at least one current source to charge the saidcapacitor, and means for selectively connecting to the electrostaticball position device relative to a given direction of the orthogonalaxes system associated with the casing, either the follower systemoutput relative to said given direction or the resistance and capacitorcircuit.

By discharging the capacitor of the resistance and capacitor circuitacross the capacitor formed by the ball and a circumpolar electrode, anelectrostatic force proportional to the square of the discharge voltageand which decreases with the latter is produced. The energy thusgenerated can be adjusted by selecting the capacitance of the capacitorand its initial charge voltage and the speed at which the forcedecreases as a function of time can be adjusted by selecting the valueof the resistance. In the case of the vertical direction, thiselectrostatic force is in the same direction as that of the weight.Then, the ball having been brought into its measuring field, anacceleration of given amplitude is produced.

According to another feature of the invention, the follower systemrelative to a given direction includes several amplifiers with differentdistributed gains and the said follower system includes means forselectively inserting one of these amplifiers in the follower systemloop, these inserting means being, themselves, controlled by theacceleration measuring apparatus connected to the amplifier having again value immediately near the gain value of the amplifier to beinserted. In other words, when the measuring apparatus relative to agiven portion of the total acceleration range reaches one end of itsmeasuring scale, it transmits a signal which causes the insertion, intothe follower system loop, of the amplifier which deals with the portionof the range adjacent to said given portion. It is thus possible todivide the measuring range into several sub-ranges each corresponding toan amplifier. As regards the follower system amplifier gain, it is onlylimited by the break-down voltage between the ball and the casing invacuum.

The micro-accelerometer may compromise, apart from the ball, a movingmass and means for applying thereto an alternating rectilinear orcircular movement. By suitably determining the weight of this mass andits movement it is possible to produce and apply to the accelerometer anacceleration of known amplitude.

The invention will now be described in detail with reference to theaccompanying drawings in which:

FIG. 1 illustrates a first model of a capacitive accelerometer accordingto the invention shown in cross-section by a plane passing through thecommon center of the ball and spherical-casing and through the center ofthe two opposite surfaces forming the cubic outer surface of the casing;

FIGS. 2a, 2b and 20, respectively, form an exploded view respectivelyshowing the casing partially cut away, two of the electrode-supportbushings and the ball of the accelerometer;

FIG. 3 is a front view of the accelerometer;

FIG. 4 is a cross-sectional view of an electrode-support bushing;

FIG. 5 is a cross-sectional view of the ball;

FIG. 6 illustrates the wiring diagram of the electrical system of theaccelerometer including the position detecting means and the positioncontrol means;

FIG. 7 is a curve showing the variation of the spacing between the balland the casing during free-fall;

FIG. 8 illustrates a perspective view of a second model ofmicro-accelerometer;

FIG. 9 is a perspective view of the open casing showing the ball withinthe casing;

FIG. 10 is a cross-sectional view of one of the electrode-supportbushings containing the position detecting electrode and the positioncontrol electrode;

FIG. 11 is a perspective view of one of the cases containing apre-amplifier connected to the position detecting electrode;

FIG. 12 illustrates the follower system line relative to the verticaldirection comprising a resistance and capacitor circuit designed forpre-positioning the ball in its operative domain;

FIG. 13 illustrates the follower chain relative to the verticaldirection comprising three amplifiers;

FIG. 14 illustrates the moving mass for calibration; and

FIG. 15 illustrates the generators of accelerations of predeterminedamplitudes.

Referring to FIGS. 1, 2a, 2b, 2c and 3, reference numeral 1 designates alight ball polished and machined with very great accuracy (typicalparticulars of a ball will be given hereafter) and reference numerical 2designates a casing with a cubic outer surface and a substantiallyspherical inner surface surrounding the ball. The casing should consist,theoretically, of the space included between a concentric outer cube andan inner sphere. In fact, for machining purposes and also to enableinsertion of the ball in the casing, casing 2 consists of a spaceincluded between an outer cube and three cosecant cylindrical bores 3the axes of which form a trihedral, the origin of which coincides withthe center of the cube and the axes of which are perpendicular to thefaces of the cube. The diameters of the three bores 3 are equal andslightly larger than the diameter of the ball. The interior of thecasing unit is, therefore, not spherical and in fact the partiallyspherical interior surface of the casing results from suitably shapedbushings fitted in the bores.

Electro-support bushings 5 consisting of a circular plate 6, acylindrical part 7, a conical part 8 and a bottom shaped in the form ofa spherical re-entrant cap 9 can be inserted into bores 3. Thus thespherical surface of the casing only consists of six spherical caps withthe remainder of the surface being cylindrical. This arrangement doesnot give rise to any serious disadvantage in the operation of theapparatus, since the casing exhibits cylindrical symmetry around eachposition detecting axis and each position controlling axis. However,since the direct potential of the ball is fixed by capacitance, fixingis improved with the increase in the capacitance of the capacitor formedby the ball and the casing. From this point of view, the second model ofcasing described in connection with FIG. 9 offers advantages over thefirst model being described at present.

A cylindrical housing 10 is machined into the spherical part of eachelectrode-support bushing. An insulating disk 11, made out of siliconoxide for example, a polar electrode 12 and a circumpolar electrode 13both made out of stainless steel for example, are inserted into thishousing. The electrodes are made to adhere to the insulating disk bymeans of a heat hardening resin and they are shaped in such a mannerthat the front surface has the shape, respectively, of a cap and a ringof spherical surface. The electrodes are equipped with connectionterminals, 14 and 15 respectively, which pass through the support part 5in insulated tubes 16 which are made out of steatite for example. Thecircular plate 6 of the electrode-support member has a diameter slightlylarger than that of the cylindrical portions 7 of this member thusforming a flange with its lower part resting on one of the surfaces ofeasing 2 and which is secured to this surface by means of screws 17.

It can be ascertained, in the structure just described, that theconducting ball 1 forms a first capacitor with the cylindrical metalwall of easing 2 and the interiors of the spherically shaped caps 9,which capacitor has of large capacitance through which the potential ofthe ball is fixed, a set of six second capacitors with the polarelectrodes, which capacitors form the position sensors and a set of sixthird capacitors with the circumpolar electrodes which capacitors formthe position control devices. The position detecting signal relative toone of the polar directions is picked-up between the two polarelectrodes relative to this direction and the position control signaltransmitted by the follower system corresponding to this detectionsignal, is applied between the two circumpolar electrodes surroundingsaid polar electrodes.

A section of the ball is illustrated in FIG. 5. It may be seen that itis hollow and that it consists of two hemispheres 18 and 18' fitted intoeach other. The leading particulars of a typical ball are as follows:

Diameter: 39.974 mm. Weight: 33.80 gr. Maximum spherical tolerance: 0.54.

The space between ball and casing, in the case where the ball and thecasing are concentric can decrease down to a few tens of ,IL. Theapplicant has made apparatuses in which the value of the gap was,respectively, p. and 10 I FIG. 6 illustrates the wiring diagram of theaccelerometer in the form of a block diagram for one coordinate of thetrihedral. There are, therefore, two other chains which are identical tothat illustrated.

The capacitors formed by electrodes 12 and 12 and ball 1 form part of ameasuring bridge including two resistances 19 and 19' and the bridge issupplied by generator 20 between the common point of the two resistances19 and 19' and the casing. The unbalanced signal of the bridge isamplified in amplifier 21 and detected in synchronous detector 22. Thedetected signal is applied to a correcting network 23 then to adifferential ampliher 24 and the two output signals, which have the formV ikv (where k is a constant and v a voltage proportional to thedisplacement of the ball) are applied to the two circumpolar electrodes13 and 13. In one example of an accelerometer made by the applicant, thefrequency of the generator 20 was 50 Hz., the power supply voltage ofthe order of 1 volt R.M.S. Voltage V can be alternating and in the samemodel of accelerometer, the R.M.S. value of this voltage V was taken tobe equal to 10 volts. Although this is not, in fact, necessary, it ispossible to separate the actuations in the three follower directions bysupplying the sensing electrodes from generators with differentfrequencies and by placing band filters corresponding to thesefrequencies before amplifiers 21.

It is also possible to obtain separation of actuation in the threedirections by supplying the electrodes corresponding to each of thethree axes with three phase voltages. Under these conditions, theaddition of the three residual potentials on the ball gives a zeroresultant.

The general expression of the electrostatic forces on the ball is:

where e and 6 have already been defined, z is the distance from thecenter of the spherical casing to the actual center of the ball and S isthe surface of the actuating electrode.

Around the operational point, the following is obtained:

6F dF AF dz+ duaz+bv (I) by effecting a change in variables dz==z anddv=v and by stating:

m=the weight of the ball;

u the output voltage of the synchronous detector 22;

k=the constant linking it to the displacement z of the ball;

u =the reference potential corresponding to the original position of theball;

ix=the signal error;

G=the gain of amplifier 24.

The follower system equations without a corrector system are, apartEquation 1, the following:

a v: a

md z

When the ball is maintained at the origin by the servomechanism, 11 :0.The solution of Equations 2 to 4 gives:

d u m (bGk a) 0 The solution of this differential equation isnondivergent for To obtain a convergent solution, it is necessary tointroduce a term of damping which is given by corrector network 23.

Referring now to FIG. 8, the micro-accelerometer comprises a casingwhich is no longer cubic but spherical 51 consisting of two hemispheres52 and 53 each with a flange 54. These two hemispheres are fixedtogether by means of screws 55 through threaded holes 56.

By taking as poles of the sphere the points where the position detectingelectrodes relative to the vertical axis are located, the flangedportions 54 are not located in an equatorial plane, that is to say thatthe poles of the hemispheres are not the poles of the casing, A lineperpendicular to the equatorial plane of the hemispheres makes angles of5445 (angle of the diagonal of a cube with the edges of the cube) withthe axes joining the centers of the diametrically opposed polarelectrodes.

Casing 51 is attached to plate 57 by means of two oblique consoles orbrackets 58 (only one of the two brackets is shown in FIG. 8), which arescrewed by one extremity to the plate and carry, at the other extremity,a groove 59 in which the edges 54 of the hemispheres are inserted andattache-d.

The two hemispheres include exterior tubes 66 and 61 which areperpendicular to their common diametral plane and which serve as amandrel during machining. One of these tubes 61 is blocked and the other60 is coupled to ionic pump 62. This pump is supported with respect toplate 57 by a bent fitting 63. It receives power supply throughconnector 64 by means of a supply line (not shown).

Sphere 51 carries six reamed drillings or bores the axes of which arealigned in pairs forming a trihedral. A bushing 65 (FIG. 10) is insertedinto each bore. The inner part of bushing 65 comprises a first recess 66and a second recess 70. Inside recess 66 is located a central electrode67 in the form of a cylinder the front surface 68 of which is given theshape of a re-entrant spherical cap and the rear surface of which isprolonged by an extension pin 69. Inside recess 7t) is located aperipheral electrode 71 shaped in the form of a ring the front profile72 of which is hollowed out to form a re-entrant spherical segment andwhich is provided with a connection pin 73. These two electrodes areinsulated from bushing 65 by means of an insulating and sealing material74 such as glass, ceramics or heat hardened resin. The two housings 66and 70 are separated by a spacer ring or guard ring 75. The purpose ofthis ring is to uncouple the capacitor between the position detectingelectrode and the ball from the capacitor between the position controlelectrode and the ball. Connection wires 69 and 73 terminate at aconnector 76. There are, therefore, six connectors 76 on the sphere,only one of them being shown in FIG. 9.

The connection between connectors 76 and the follower system circuits iseffected, in the case of each follower system direction by means ofbrackets (FIG. 11). There are three of these brackets 81, 82, '83 andeach consists of two parallel tubes 77 and 78 connected by a fiat box79. This box contains the preamplifier from which each follower systemstarts (this pre-amplifier forms part of amplifier 21 in FIG. 6). Tubes77 and 78 are provided with slots 84 in which the connectors 76penetrate. These are attached to plate 57 by brackets such as bracket85.

Referring now to FIG. 12, a follower chain is shown the differentcircuits of which are designated by the same reference numerals as inFIG. 6 and furthermore, there are represented a direct current source 39connected in parallel to the terminals of a capacitor 40, to aresistance 41 and to the position control electrodes 13-13 relative tothe vertical axis. The acceleration measuring apparatus connected to thedifferential amplifier 24 is designated by the reference numeral 100. Afirst switch 42 is placed in series with the current source 39; a secondswitch 43 is placed in series with resistance 41; a third switch 44 isinserted between the pre-positioning circuit and the ball positioncontrol electrodes 1343 and a fourth switch 45, finally, is insertedbetween the output of amplifier 24 in the follower chain and the saidposition control electrodes.

The various switches are controlled by a programming unit 46 inaccordance with the following program starting from instant 1 when thecapsule in which the microaccelerometer is placed is released.

(a) before instant t when the capsule is released: closing of 42 for asufficient time to charge capacitor 40 and thereafter opening 42 (b)instant (t e): 44 closes (c) instant (t -H): 43 closes (d) instant (t -H43, 44 open; 45 closes Since the follower system loop must only beclosed when the ball is within its measuring range, programming unit 46only passes to stage B when the measuring unit is, itself, within itsmeasuring range.

In the second pre-positioning system, the sensitivity is modified duringfree-fall in such a manner as to pass from g to 10 g during the fraction(t t of time spent in free-fall. By referring to FIG. 13, three circuits47, 48, 49 have been shown which each include a corrector circuit 23 2323 respectively and a differential amplifier 24 24 24 respectively.These amplifiers have different gains and an operational range of 10 Thefirst amplifier 24 serves for the acceleration range g10 g, the second24 for the acceleration range 10- g10 g and the third 24 for theacceleration range 10- g-10* g. .Since the forces applied to the ballare proportional to the square of the voltages,

the three amplifiers have gains with ratios of /1O Amplifier 24 has again such that it is capable of supplying at maximum, an output signalof the order of 5000 volts and amplifiers 24 24 can deliver outputsignals of the order of 150 and 5 volts respectively.

The three differential amplifiers 24 24 24 are associated with measuringunits 99 99 99 The measuring units are connected to a programming unit86 and are provided with means for indicating that they are at the lowerlimit of their measuring range. Such means may consist of an electricalcontact.

A three position switch 50, which is controlled by programming unit 86is inserted between the output of the synchronous detector 22 andassemblies 47, 48, 49. At the moment of release t the switch is inposition 50, then it passes, successively, to position 50 and 50 at themoments when measuring units 99 99 arrive, respectively, at the lowerlimit of their measuring range in such a manner that, at instant t onlyassembly 49 is in service;

FIG. 14 represents the vibrator designed to give the micro-accelerometeran acceleration of a known value directed along one of the directions ofthe three orthogona'l planes associated with the casing. It is attachedto a plate 87 which can be secured by posts 88, to plate 57 of themicro-accelerometer in such a manner that the vibrational direction ofthe vibrator is in alignment with the direction of themicro-accelerometer under consideration. Posts 88 are screwed into thethreaded holes in plate 57 one of which 95 is shown in RIG. 8. Thevibrator includes a magnetic circuit in the form of a pot 89, an innerwinding 90, a plunger core manufactured out of a suitable magneticmaterial 91 coaxial with winding 90 and a pole piece 92 attached to theheel piece in front of the plunger core. Core 91 is attached to the heelpiece by means of two flexible disks 93 and 94. When the coil issupplied with alternating current, core 91 vibrates at the frequency ofthis current. The amplitude of the acceleration is measured at terminals98 of the capacitor formed by pole piece 92 and a plate 101 carried bydisk 94.

For example, if it is assumed that the weight of core 91 is 1 gr., thatthe weight of the capsule containing the micro-accelerometer is 100 kg.and that the frequency of the power supply current is 1.5 Hz., theacceleration 7 applied to the micro-accelerometer by the core subjectedto an acceleration '7 is If it is desired to apply an acceleration of g,for example, to the microaccelerometer, it will be necessary that'y1=10" in. see? that is to say that the amplitude of the vibration ofthe vibrator is Switching of current into winding 90 is controlled atinstant t by programming unit 46 or programming unit 86 which closescontact 96 connecting the alternating current source 97 to winding 90 Inorder to calibrate the accelerometer along the three axes of thetrihedron associated with the casing, two other vibrators, identical tothat which has just been described, are associated with themicro-accelerometer. FIG. illustrates the arrangemnt of plates 87, 102,103 of these vibrators relative to micro-accelerometer plate 57 in sucha manner that the vibrational directions are aligned with the directionsof the three orthogonal axes of the trihedron associated with thecasing. It can be seen that plate 102 is attached to plate 57 by meansof a angle-iron 104 and two posts 105 and that plate 103 is attached toplate 57 by means of a angle-iron 106 and two posts 107.

FIG. 7 shows a curve 26 illustrating the recording of the relativemovement of the ball with respect to the casing during a test infree-fall. It may be seen that passage to zero takes place approximatelyfifteen milliseconds after the beginning of the experiment.

What we claim is:

1. Electrostatic accelerometer providing an integrated measure ofacceleration along three orthogonally arranged axes comprising a hollowcasing substantially spherical at its inner side having electricalconductivity, a spherical acceleration sensing mass having electricalconductivity and freely floating in said casing in gravity freecondition when not subjected to acceleration, three opposed pairs ofpolar electrodes isolated from the casing. and located in orthogonalrelation on the inner casing surface to provide the axes of theaccelerometer, said polar electrodes forming with the spherical massouter surface six first capacitors adapted to detect the position ofsaid mass, three opposed pairs of circumpolar electrodes isolated fromthe casing and respectively surrounding the polar electrodes, saidcircumpolar electrodes forming with the spherical mass outer surface sixsecond capacitors adapted to control the position of said mass, theinner surface of the casing and the outer surface of the spherical massforming a third capacitor having. a capacitance highly larger than thatof the first and second capacitors, means for applying an alternatingcurrent between said casing and respectively said polar electrodes,means connected to said polar electrodes for providing output signalsdepending on the displacement of the mass along the orthogonal axes froma centered position and associated with said axes, means for detectingand amplifying said signals and means for applying the detected andamplified signals associated with the orthogonal axes respectively tothe circumpolar electrodes corresponding to said axes.

2. Electrostatic accelerometer as claimed in claim 1 in which saidposition detecting polar electrodes and position controlling circumpolarelectrodes are separated by guard rings brought to the electricalpotential of the spherical mass.

3. Electrostatic accelerometer as claimed in claim 1 in which the casingcomprises two hemispheres connected to each other by flanged portionsarranged in a common diametral plane and the polar and circumpolarelectrodes are located in each hemisphere at the intersection points ofsaid hemisphere with orthogonal axes centered at the end of the radiusof the hemisphere perpendicular to the common plane and forming withsaid radius an angle equal to that formed by the edges of a cube withits diagonal.

4. Electrostatic accelerometer as claimed in claim 1 comprising meansfor applying a vacuum to the space comprised between said hollow casingand said spherical mass.

5. Electrostatic accelerometer providing an integrated measure ofacceleration along three orthogonally arranged axes in a given range ofaccelerations divided into sub-ranges comprising a hollow casingsubstantially spherical at its inner side having electricalconductivity, a spherical acceleration sensing mass having electricalconductivity and floating freely in said casing in gravity freeconditions and when not subjected to acceleration, three opposed pairsof polar electrodes isolated from the casing and located in orthogonalrelation on the inner casing surface to provide the axes of theaccelerometer, said polar electrodes forming with the spherical massouter surface six first capacitors adapted to detect the position ofsaid mass, three opposed pairs of circumpolar electrodes isolated fromthe casing and respectively surrounding the po lar electrodes, saidcircumpolar electrodes forming with the spherical mass outer surface sixsecond capacitors adapted to control the position of said mass, theinner surface of the casing and the outer surface of the spherical massforming a third capacitor having a capacitance highly larger than thatof the first and second capacitors, means connected to the polarelectrodes for applying an alternating current between said casing andrespectively said polar electrodes, means for providing output signalsdepending on the displacement of the mass along the orthogonal axes froma centered position and associated With said axes, means for detectingsaid signals, a plurality of amplifiers connected between said detectingmeans and said circumpolar electrodes and having different gains forminga discrete sequence of gains for amplifying said detected signals, meansfor selectively operating one of said amplifiers between said detectingmeans and respectively the circumpolar electrodes and means formeasuring the output signals of said amplifiers, whereby the outputsignal of a given amplifier measures the acceleration acting upon thesensing mass in an acceleration subrange depending on the gain of theinserted amplifier.

6. Electrostatic accelerometer as claimed in claim 5 in which the meansfor measuring the output signal of a given amplifier connected betweenthe detecting means and the circumpolar electrodes comprises means forgenerating a switching signal when said measuring means are at one endof the sub-range thereof and means for controlling by said switchingsignal the selective operating means, thereby inserting between thedetecting means and respectively the circumpolar electrodes theamplifier having, in the discrete sequence of gains, a gain nearlyadjacent to that of said given amplifier.

7. Electrostatic accelerometer providing an integrated measure ofacceleration along three orthogonally arranged axes comprising a hollowcasing substantially spherical at its inner side having electricalconductivity, a spherical acceleration sensing mass having electricalconductivity and floating freely in said casing in gravity freecondition and when not subjected to acceleration, three opposed pairs ofpolar electrodes isolated from the casing and located in orthogonalrelation on the inner casing surface to provide the axes of theaccelerometer, said polar electrodes forming with the spherical massouter surface six first capacitors adapted to detect the position ofsaid mass, three opposed pairs of circumpolar electrodes isolated fromthe casing and respectively surrounding the polar electrodes, saidcircumpolar electrodes forming with the spherical mass outer surface sixsecond capacitors adapted to control the position of said mass, theinner surface of the casing and the outer surface of the spherical massforming a third capacitor having a capacitance highly larger than thatof the first and second capacitors, means for applying an alternatingcurrent between said casing and respectively said polar electrodes,means connected to the polar electrodes for providing output signalsdepending on the displacement of the mass along the orthogonal axes froma centered position, means for detecting and amplifying said signals, aresistance and capacitor discharge circuit, a current source forcharging said capacitor of said latter circuit and means for selectivelyapplying to the circumpolar electrodes the detected and amplifiedsignals associated with the orthogonal axes and said resistance andcapacitor discharge circuit.

8. Electrostatic accelerometer providing an integrated measure ofacceleration along three orthogonally arranged axes comprising a hollowcasing substantially spherical at its inner side having electricalconductivity, a spherical acceleration sensing mass having electricalconductivity and floating freely in said casing in gravity freecondition and when not subjected to acceleration, three opposed pairs ofpolar electrodes isolated from the casing and located in orthogonalrelation on the inner casing surface to provide the axes of theaccelerometer, said polar electrodes forming with the spherical massouter surface six first capacitors adapted to detect the position ofsaid mass, three opposed pairs of circumpolar electrodes isolated fromthe casing. and respectively surrounding the polar electrodes, saidcircumpolar electrodes forming with the spherical mass outer surface sixsecond capacitors adapted to control the position of said mass, theinner surface of the casing and the outer surface of the spherical massforming a third capacitor having a capacitance highly larger than thatof the first and second capacitors, means connected to the polarelectrodes for applying an alternating current between said casing andrespectively said polar electrodes, means for providing output signalsdepending on the displacement of the mass along the orthogonal axes froma centered position and associated with said axes, means for detectingand amplifying said signals, means for applying the detected andamplified signals associated with the orthogonal axes respectively tothe circumpolar electrodes corresponding to said axes and means forsubmitting the accelerometer to predetermined accelerations.

References Cited UNITED STATES PATENTS 2,942,479 6/ 1960 H-olmann.3,272,016 9/1966 Mullins 73-517 3,295,379 1/1967 Jensen et al 74-5 XRJAMES J. GILL, Primary Examiner.

